Dielectric materials such as silicon oxides are widely used in the manufacture of semiconductor devices. These materials find use not only as final passivation coatings for completed devices but also as intermediate insulatng layers for multi-layer devices. Thermally grown silicon oxides are typically employed as dielectric films, e.g. a gate dielectric, in semiconductor devices such as EAROMs (electrically alterable read only memory), MOSFETs (metal-oxide-semiconductor field effect transistor), capacitors and the like. Thermal oxidation methods, which produce the best silicon oxide films, are usually carried out by placing silicon in an oxygen ambient at temperatures between 800.degree. and 1200.degree. C.
These temperatures can be tolerated by silicon substrates, but in some situations the above temperatures are too high for the substrate. For example, in applications where the dielectric film to be formed is part of a semiconductor device being fabricated over a glass substrate, lower processing temperatures are required. Specifically, in the area of liquid crystal displays, thin film transistors are fabricated over glass substrates which have a softening point of about 650.degree. C. Therefore, to thermally grow an oxide from silicon over a glass substrate, the temperature of the substrate in the oxygen ambient must be kept at about 600.degree. C. or below. This temperature requires about 120 hours to grow a 60-70 nanometers (nm) thick layer of silicon oxide. This is not practical.
Silicon oxides, aluminum oxide, and silicon nitride dielectric layers can be deposited at temperatures below 600.degree. C. by glow discharge and other chemical vapor deposition (CVD) techniques, in a fraction of the time required for thermal growth, e.g. a few hours or less. However, the dielectric quality of the deposited film is poor compared to the thermally grown silicon oxide. Specifically, the transistor turn on voltage for deposited silicon dioxide films is typically unstable which is not true for thermally grown films. It is believed that charge traps within the film and/or at the silicon-dielectric interface cause the film to accumulate a charge upon the initial applications of voltage. Subsequent applications of voltage are characterized by a shift in the turn-on, or threshold, voltage of 5 to 10 volts or more, compared to little or no shift in the threshold voltage for thermally grown oxide.
Further, in applications requiring a dielectric layer over hydrogenated amorphous silicon, it is desirable to keep processing temperatures below about 400.degree. C. This is because the semiconductive properties of amorphous silicon change at temperatures above 400.degree. C., probably due to a loss of hydrogen from the film at elevated temperatures.
It would be desirable therefore to have a method of producing a dielectric film at a temperature below 600.degree. C. and preferably below 400.degree. C. wherein the rate of film formation is substantially increased over thermally grown films in that temperature range without a substantial sacrifice in dielectric stability.